Method of growing strain-free single crystals



y 6, 1970 J. R. PASTORE 3,514,265

METHOD OF GROWING STRAIN-FREE SINGLE CRYSTALS Filed April 5, 1967AUTOMATIC LOWERING MECHANISM FURNIIPCA i i I VACUUM SYSTEM 22 GRAPHITEUCIBLE [8 SOLID CHARGE MELT GROWTH INTERFACE -TO GROWING CRYSTAL I R.F.NUCLEATION POINT ,7 GENERATOR INVENTOR,

JOHN R. PASTORE.

0 W ATTORNEYS United States Patent 3,514,265 METHOD OF GROWINGSTRAIN-FREE SINGLE CRYSTALS John R. Pasture, Long Branch, NJ assignor tothe United States of America as represented by the Secretary of the ArmyFiled Apr. 5, 1967, Ser. No. 628,795 Int. Cl. B01j 17/24; C01g 11/00;C01f 11/22 US. Cl. 23301 2 Claims ABSTRACT OF THE DISCLOSURE Smalldiameter, strain-free single crystals are grown from a melt of powder ina high density crucible having a high ratio of outer diameter to innerdiameter by gradually lowering the crucible through an RF coil containedin an evacuated furnace.

BACKGROUND OF THE INVENTION This invention relates in general to amethod of growing strain-free single crystals, and in particular to amethod of growing strain-free small diameter single crystals in laserrod form.

Low threshold laser crystals must be highly perfect and strain free. Forcontinuous wave operation, further threshold lowering and more effectivecooling is achieved by the use of small diameter crystals. A methodpresently used for the production of laser rods involves the growth of alarge diameter crystal which is slowly grown over a period of manyhours, cooled very slowly, annealed for days in special ambients toremove growth-induced strains, and finally machined to size. Such rodsare still prone to fracture because of residual strain during theinitial cool-down prior to laser oscillation tests.

SUMMARY OF THE INVENTION The general object of this invention is toprovide a method of growing a small diameter strain-free single crystal.A particular object of this invention is to provide a method of growingthese single crystals in laser rod form. A still further object of thisinvention is to provide a method of growing a small diameter strain-freesingle crystal which eliminates the time consuming re quirement ofannealing or after heating, necessary in conventional growth methods.

It has now been found that the. foregoing objectives can be attained bya method utilizing a high density, high purity crucible as hereinaftermore fully described. According to the invention, a high density, highpurity crucible is filled with the powder to be grown. The filledcrucible is then capped with a vented plug and positioned within an RFheating coil contained in an evacuated furnace. The powder is melted andthe temperature increased to establish thermal equilibrium. Crystalgrowth is then started by lowering the crucible through the RF coil at aprescribed rate, the lowering of the crucible being stopped after themelt has crystallized.

The invention can be best understood by referring to the accompanyingdrawing wherein there is shown a cross section of suitable apparatus forcarrying out the invention.

BRIEF DESCRIPTION OF THE DRAWING Referring to the drawing, a furnace isfitted with a water cooled jacket 12 and evacuating means 14. Positionedwithin the furnace 10 in approximately the center thereof is a watercooled RF coil 16. A high density, high purity crucible 18 capped with avented plug 20 is hung by crucible hanger 22 in such manner as to bepassable through RF coil 16, as shown. An automatic 3,514,265 PatentedMay 26, 1970 ice lowering mechanism (indicated by the arrow) isoperat1vely connected with crucible hanger 22 and serves to lower thecrucible 18 through the RF coil 16.

DESCRIPTION OF THE PREFERRED EMBODIMENT In a specific embodiment of theinvention, a high density, high purity graphite crucible 18 having anouter diameter of about one inch and an inner diameter of about inch toform a bore is filled with a mixture of calcium fluoride powder orcadmium fluoride powder with 0.1 mole percent by weight of the total mixof dysprosium fluoride. The crucible bore is then capped with a ventedplug 20 to reduce the escape of calcium fluoride vapors and the entireassembly is placed in furnace 10 substantially as shown, the furnacebeing evacuated to 1O- to 10- torr. After the charge has been melted,the temperature is increased another C. After thermal equilibrium isestablished, the growth of a single crystal is started by lowering thecrucible through the RF coil 16 at a rate of about inch to about inchper hour. As shown, nucleation occurs at a point below RF coil 16. Afterthe melt has crystallized, the lowering of the crucible 18 is'stopped.The temperature of the grown single crystal is decreased from themelting point to 300 C. by increments of about 100 C. per 5 minutes toeliminate the occurrence of strain in the last solidified portion of thecrystal; then all power to the RF coil 16 is turned off. After two hoursthe furnace 10 is blanked off from the vacuum system 14 and the crucibleis cooled at room temperature before its removal. The crystal is theneasily freed from the crucible by gentle tapping.

In the method as described above, the thick walled graphite crucibleprovides a large thermal mass which allows sudden fluctuations of energyinput (due to line voltage variations) to be damped out before theireffect can reach the growing crystal. The large thermal mass and the lowthermal conductivity of the graphite crucible, in addition to the smallcrystal diameter, allow a planar growth interface which is the idealsituation for growth of single crystals. During growth, as any plane ofthe crucible passes through the RF coil, it continues to be heated bythe RF field. As the flux lines fan out rapidly, the heating of thecrucible tapers off gradually away from the coil. This tapering heatinput helps to balance the radiative loss and produces a gradual,instead of an abrupt, temperature change in the crystal beyond therecrystallization isotherm. This residual RF heating, the large thermalmass, and low thermal conductivity of the thick wall graphite allcontribute to the very gradual temperature change along the crucible.The very gradual temperature decrease along the crucible means that theisotherms are perpendicular to the crucible axis and decrease intemperature away from the RF heater. As a result, the radial heat lossfrom the crystal is almost eliminated and the major heat flow from thecrystal is essentially in an axial direction away from the growthinterface. This one dimensional distribution of heat has been shown tobe the most desirable condition for cooling a grown crystal. Thebeneficial effect of the method of the invention is verified bysubstituting a thin-wall graphite crucible for the thick wall andkeeping all other parameters constant for a growth run. Though thecrystals grown in the thin-wall crucible are predominantly single, noneare strain free.

The single crystals are grown according to the invention in a matter ofhours. Except for cutting and polishing of the ends, they are in laserrod form. For growth of other compounds or elements, a crucible ofsuitable RF susceptor type material and appropriate ambient are chosen.The length of the particular crucible used depends upon the length ofthe crystal desired and the limitations of the growth apparatus. Theratio of the outer diameter to the inner diameter of the particularcrucible used should be at least 8 to 1. Of course, the crucible usedmust not react with the material that you are attempting to grow andmust not stick to it after freezing.

Then, too, rather than cool the crystal by the 100 C. per 5 minuteincrements as shown in the embodiment, a more favorable method would bean automatic and continuous temperature decrease over a specified periodof time. The ideal case, if furnace dimensions allowed, would be to havea crucible of sufiicient length, possibly 24 inches, with the crystal inthe bottom lowered three inches through the RF coil until the top of thegrown crystal was at 300 C. or less. Depending on the crystalline oramorphous material to be grown, an appropriate atmosphere is chosen,such as an inert gas or nitrogen. The RF coil can also be of sufficientlength so that all the material not nucleated will be in a molten state.

Several methods were used to evaluate the quality andproperties of thecrystals grown. All cleaved easily on the {111} plane and resulted inflat surfaces free of :urvature which is a good indication of theabsence of ;train. When the crystals were viewed through crossedpolarizers no evidence of strain was seen. I

The crystals were tested for lineage and singularity, by neans of ascanning Laue X-ray diffraction method, in which the crystal and filmare translated together normal a stationary X-ray beam in the Lauearrangement. Fhe resulting photograph shows a series of lines which treparallel to the direction of crystal translation. The .traightness ofthese lines is a measure of the crystalline )erfection. Imperfectionssuch as lineage, mosaic struc ure, and grain boundaries, will berevealed as irregulariies and breaks in the lines. Representativepatterns show hat the crystals grown by this technique have a highlegree of perfection.

Dislocation counts for the unactivated crystals grown vy this methodwere 5.3 10 /sq. cm. compared to .4X10 /sq. cm. for the optical gradeCaF (Harshaw Zhemical Co.) starting material.

A CaF zDy crystal, grown according to the invention :s ends paralleled,polished, and silvered, then irradiated with electrons to convert the Dyto By was tested or laser oscillations. The pulsed threshold for thecrystal t 77 K. in a concentric head was 28 joules input to an T524lamp.

Seeded 111 crystals have been grown by this lethod. The easy cleavage{111} plane is perpendicular the crystal axis. Thus the ends of thecrystal can be leaved ofl easily with a razor blade resulting in a laserad with a Fabry-Perot geometry.

Consequently a suitably activated crystal, with cleaved nds, mounted inan outside mirror type laser head can e used for laser oscillationsalmost upon removal from 1e growth crucible.

This would eliminate the many steps and many hours :quired for slowcrystal growth, slow cooldown, anneal- 4 ing, paralleling and polishingthe ends and polishing the barrel of the laser crystal.

The foregoing description is to be considered merely as illustrative ofthe invention and not in limitation thereof.

What is claimed is:

1. Method of growing small diameter strain-free single crystalsincluding the steps of:

(A) filling a high density, high purity graphite crucible having anouter diameter of about one inch and an inner diameter of about 4 inchwith a mixture of calcium fluoride with about 0.1 mole percent by Weightof dysprosium fluoride,

(B) capping the filled crucible with a vented plug,

(C) positioning the filled crucible within an RF heating coil containedin an evacuated furnace,

(D) melting the crystalline powder,

(E) increasing the temperature to establish thermal equilibrium,

(F) starting crystal growth by lowering the crucible through the RF coilat a rate of about A; inch to about inch per hour, and

(G) stopping the lowering of the crucible after the melt hascrystallized.

2. Method of growing small diameter strain-free single crystalsincluding the steps of:

(A) filling a high density, high purity graphite crucible having anouter diameter of about one inch and an inner diameter of about inchwith a mixture of calcium fluoride with about 0.1 mole percent by Weightof dysprosium fluoride,

(B) capping the filled crucible with a vented plug,

(C) positioning the filled crucible within an RF heating coil containedin an evacuated furnace,

(D) melting the crystalline powder,

(E) increasing the temperature to establish thermal equilibrium,

(F) starting crystal growth by lowering the crucible through the RF coilat a rate of about inch to about inch per hour, and

(G) stopping the lowering of the crucible after the melt hascrystallized.

References Cited UNITED STATES PATENTS 3,033,659 5/1962 Fischer 23-2953,139,653 7/1964 Orem 22-57 3,243,267 3/1966 Piper 23301 3,273,9699/1966 Sirgo 23273 OTHER REFERENCES Chemie.-lng.-Techn., vol. 28, No. 5(1956), p. 358.

NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US.Cl. X.R. 23-88, 273

